Focus detection system with zero crossing detection for use in optical measuring systems

ABSTRACT

The invention concerns a focus detection circuit for an optical trigger probe. The probe has a laser light source, a beam splitter and a focusing lens system. The probe produces a light beam focused to a point at a distance d from the probe. Light reflected from the surface is reflected from the beam splitter onto two pairs of photosensitive detectors. The signals from the two outer detectors are added as are the signals from the two inner detectors, and the two sums are subtracted to provide a focus error signal which is passed to a zero crossing detector. When the light beam is focused on the surface, the difference between the sums of the detector signals will be zero and the zero crossing detector circuit emits an output pulse. To eliminate false trigger signals when the probe is some distance from the workpiece and the reflected light is minimal, a validating signal is generated by measuring the intensity of the relected light at a detector. The validating signal so produced, based upon the reflected light, is passed to an AND gate along with the output pulse. The AND gate only transmits a trigger signal when a validating signal and an output pulse are present at the same time.

BACKGROUND OF THE INVENTION

The present invention relates to a focus detection circuit for detectingwhen the focal spot of a focussed light beam is in focus on a surfacetowards which the light beam is directed.

It is known in optical scanning probes to focus a light beam from asemiconductor laser onto a surface using a movable objective lens. Thereflected light is re-focussed inside the probe onto a focus detectorconsisting of a pair of photo-diode detectors, the output of whichcontrols a lens tracking mechanism which maintains the focal point onthe surface. The design is such that a displacement of the objectsurface in the direction of the light beam results in a change in thedistribution of light energy falling on the photo-diodes. This providesa focus error signal which is used to move the lens in a direction suchas to reduce the focus error signal to zero, at which point the focalpoint of the light beam is back on the surface.

The above-described tracking mechanism relies on the fact that the focalpoint is always at the same distance from the lens, when using acollimated or coherent light source, so that the movement of the lenswhich is required to bring the light spot back into focus on the surfaceequates to the movement of the surface.

It is also known to provide an optical trigger probe (for example aprobe described in an article in OEM DESIGN published in Oct. 1986)which may be moved towards a surface and provides a signal when a beamemitted by the probe comes into focus on the surface.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a focus detectionsystem suitable for use with an optical trigger probe.

According to the present invention there is provided an opticalmeasuring system comprising a probe having a housing, a light source anda focussing device mounted within the housing for producing a light beamfocussed at a point outside the housing, and a focus detection systemfor receiving light reflected from a surface towards which the lightbeam is directed, charactrised in that the light source and thefocussing device are mounted in fixed relationship within the housing,the focus detection system including detector means for producing twooutputs indicative of the amount of said reflected light falling ondifferent parts thereof and means receiving said two outputs and forproducing a focus error signal which has a zero value when the lightbeam is in focus on the surface and a non-zero value when the light beamis out of focus on the surface, and wherein the focus detection systemfurther includes a zero crossing detection circuit for detecting thezero crossing points of the focus error signal and for producing anoutput each time a zero crossing is detected.

In such a system the focal point is always a fixed distance from thelens, and provided that the lens position is accurately known within theprobe body, and the probe position is accurately known from the scalesof a measuring machine on which the probe is mounted, the position ofthe surface along the line of the light beam can be accuratelydetermined by noting the machine scale readings when the output signalis produced.

The above described focus detection system however has a range limitedto a few tens of microns. Outside this range the focus error signaltends towards zero, and any noise in the system due, for example, tovibration of the probe, or stray light within the probe, can give riseto large numbers of zero crossings when the reflected light level fromthe surface 12 is low or zero. Thus the zero crossing detector circuitmay provide a large number of false trigger signals when operatingoutside of this range.

A probe with a focus detector system as described above can be used fordetecting small variations of shape or in surface detail of a workpieceprovided that the position of the surface remains within the workingrange of the probe.

In order to adapt such a probe for use with a co-ordinate measuringmachine in which the probe is driven towards a surface from a point somedistance away, and outside the normal operating range of the focusdetector system, it is necessary to determine which of the triggersignals being produced from the focus detection system are false andwhich are genuine.

According to a feature of the invention the focus detection systemincludes a validating circuit which determines the intensity of thereflected light falling on the detectors and which produces a validatingsignal only when the intensity of the reflected light received by thedetectors is greater than a pre-determined level thus indicating that agenuine focus detection signal has occurred.

By this means false trigger signals can be filtered out and the onlytrigger signals which are passed to the machine are those which areaccompanied by a validating signal.

In a preferred embodiment of the invention the validating circuitdetermines both the intensity of the reflected light received by thefocus detection circuit and the distribution thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates diagrammatically the components of an optical triggerprobe according to the present invention,

FIG. 2 shows the shape of the focus error signal produced by thedetectors in the probe of FIG. 1 as the focal point of the light beammoves through a surface,

FIG. 3 shows one embodiment of a validating circuit for use with theprobe of FIG. 1

FIG. 4 shows an alternative embodiment of a validating circuit for usewith the probe of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2 there is shown a probe having a housing 2within which are mounted, a light source 4, a beam splitter 8, and afixed focussing lens system 10. The light source 4, which may be a laserdiode, produces a light beam 6 directed towards the beam splitter 8which may be of any known type. At least a portion of the beam istransmitted through the beam splitter to the lens system 10 whichfocusses the beam to a point at a distance d from the probe. Distance dis the stand off distance from the probe to a workpiece 12 on which theprobe performs measuring operations.

The beam splitter 8 forms part of a focus detection system mountedwithin the housing and indicated generally by the reference 13. Thefocus detection system also includes a prism 14 and two pairs ofphoto-diode detectors 16, 18, the individual detectors A, B and C, D ofeach pair being separated by spaces S1 and S2. The beam splitter 8 hasan inclined surface 9 which transmits the light beam from the lightsource to the lens system 10, but reflects light reflected back from thesurface 12 at right angles towards the photo-diode pairs 16 and 18. Thereflected light from the beam splitter 8 passes through the prism 14which splits it into two beams directed onto the photo-diode pairs. Thediagrammatic representation given in FIG. 1 shows a ray diagram of thebeam in the focussed condition.

The arrangement is such that when the light beam 6 is focussed onto thesurface 12, the reflected light will be focussed onto the spaces S1 andS2 between the photo-diodes in each pair. The light spots formed by theprism in the spaces S1 and S2 are arranged, in this focussed condition,to overlap the edges of the two detectors A, B and C, D equally on bothsides of both of the spaces. When relative movement between the surface12 and the probe causes the focal point of the light beam to move to aposition above or below the surface 12, the two beams leaving the prism14 will become more or less divergent, and the spots formed by them onthe photo-diode pairs will both move either more towards the outer twodetectors A and D, or towards the inner two detectors B and C. Theresult is that the amount of light received by the outer two detectors Aand D will either increase or decrease and at the same time the amountof light received by the inner two detectors B and C will respectivelydecrease or increase.

The outputs of the inner two detectors B and C are connected at asumming junction 20 and the outputs of the outer two detectors A and Dare connected at a summing junction 21. The outputs of the two summingjunctions 20 and 21 are subtracted at a further junction 22 whichproduces an output 23 in the form shown by the curve 24 of FIG. 2. Thedistance x, between the two peaks of the curve, gives the working rangeof the detector as the focal point of the light beam moves up and down,and the amplitude of the peaks y, gives the focus error signal. A zerocrossing detector circuit 25 (known per se) is used to determine thepoint 0 on curve 24 in the working range of the detectors at which thedifference in the outputs of the outer and inner detectors is zero,which indicates that the beam 6 is focussed on the surface 12. The zerocrossing detector circuit produces an output 26 in the form of a pulseevery time a zero crossing is detected.

However, the optical trigger probe in operation will be mounted on amachine for detecting and for measuring the surface 12. The focal pointor spot of the probe will thus often be outside the working range x ofthe detectors since this is limited to a few tens of microns of relativemovement between the surface 12 and the probe.

It can be seen that as relative movement between the probe and thesurface causes the surface to move further away from the focal point ofthe light beam, the reflected light becomes more diffused, and the spotsformed by the prism 14 on the detectors 16 and 18 become rapidly largerand less intense until they cover both detectors and the differencesignal from junction 22 tends to zero. Ignoring any effects of straylight reflections on the detectors this would give a false triggersignal from the zero crossing detector at point x₁ or x₂ outside theworking range of the detectors 16, 18. In fact when the probe is somedistance from the workpiece the reflected light level is zero andoccurrence off a zero crossing signal from the detector 25 becomesunpredictable.

In order to avoid this problem a validating circuit is introduced intothe probe circuitry and which is arranged to determine when the zerocrossing point on curve 24 lies inside the working range of thedetectors and outputs a signal accordingly.

One simple method of producing a validating signal would be to add theoutputs from all of the four detectors A, B, C, D and compare them witha minimum pre-determined threshold value.

A preferred method of producing a validating signal is shown in FIG. 3.The four focus detectors A, B, C, D are mounted on a largerphoto-sensitive guard detector 40 which will be illuminated by lightspilling around the focus detectors when the light spots generated bythe prism become enlarged as the surface 12 moves away from focus. Theguard detector 40 forms part of a validating circuit and provides a"noise" signal indicative of the intensity of the reflected light andother stray light near the detectors.

The focus detection system will only operate correctly when the lightfrom the prism forms two defined spots on the detector pairs 16, 18. Ifthis is not the case, i.e. when the light beam 6 is far from beingfocussed on the surface 12, the guard detector and the detector pairswill be weakly illuminated by the reflected light. The ratio of the sumof the signals from the detector pairs 16, 18 and the noise signals fromthe guard detector, which will be referred to as the signal to noiseratio, will be proportional to the areas of the detectors.

As the probe approaches focus, the intensity of diffuse illuminationacross all of the detectors will increase, and the ratio of the signalsdescribed above will remain constant until the point is reached wherethe two spots from the prism begin to be defined on the detector pairs16, 18. The output from the guard detector will then start to drop whilethe output from the detector pairs increases. The threshold ratio can beset at the level at which the detector pairs 16, 18 are within theirworking range and a validating signal generated when this level isreached. Only those zero crossing signals developed while the validatingsignal is present will then be sent to the machine as true probe triggersignals.

On receipt of a true probe trigger signal the machine will be braked tostop further movement of the probe towards the surface 12. Braking mustbe achieved within the stand off distance d.

In FIG. 3 the remainder of the validating circuit is shown as includinga divider 42 which receives an input G from the guard detector and aninput F from a summing junction 44 which represents the sum of the fourfocus detectors A, B, C and D. The divider provides an output VR equalto the ratio of inputs F:G. The output VR of the divider is passed to acomparator 46 which also receives a reference voltage V ref 1, whichprovides a threshold value. The comparator 46 outputs the validatingsignal SV when the ratio F:G rises above the threshold value.

As described above the signals SV from the validating circuit and thesignal 26 from the zero crossing detector are arranged to be positiveand are passed to an AND gate 48 which produces a validated triggersignal only when signals SV and 26 are present at the same time.

However, when the probe is completely removed from the presence of theworkpiece there will be no reflected light received by the detectors 16,18. Under these circumstances any stray light falling on the detectors16, 18 and on the guard detector will be unpredictable and may be suchas to provide a signal to noise ratio greater than the threshold value.

Several modifications may be made to obviate this problem. Onemodification is shown in dotted lines in FIG. 3. A further comparator 50is provided to which is connected both the signal F from the summingjunction 44 and a further reference voltage Vref2. The comparator 50outputs a signal 52 when the ratio of the signal F to voltage Vref2 isgreater than a pre-determined threshold indicating that a positivesignal has been received from the detectors 16, 18 in excess of anynoise signal. The output signal 52 is passed as a third input to the ANDgate 48 as a further verification that the signal from the detector 25is a true signal.

Alternatively, bias signals can be added to each of the signals F and Gwhich are greater than the anticipated noise levels. The bias signalsare in a ratio such that when there is no reflected light from theworkpiece surface, the addition of any noise signals to the bias signalswill not cause the value of the ratio F:G to exceed the threshold value.The bias signals may be generated externally of the probe or may bederived from the amount of stray light from the source 4 which isreflected onto the detectors. This stray light would affect the guarddetector and the detectors 16, 18 in the ratio of their areas.

FIG. 4 shows an alternative method of producing a validating signal. Afurther beam splitter 30 is provided in the path of the reflected lightfrom the surface. This is arranged to deflect a known portion of thereflected light onto a second detector system in a beam 31 whiletransmitting the remainder to the beam splitter 8. The second detectorsystem which forms part of the validating circuit V, consists of asingle photo-diode detector 32 arranged to receive all of the light inthe beam 31 from the beam splitter 30. The output 34 of the detector 32provides a measure of the total light intensity which is directed fromthe mirror surface 9 towards the detectors 16, 18. The output 34 iscompared with a reference voltage 36 in a comparator 38, and the output39 of the comparator is the validating signal which indicates when theoutput 34 is above the threshold. This indicates that the intensity oflight falling onto the detectors 16, 18 is within the working range ofthe detectors. In order to determine the distribution of the lightfalling on detector 32, the detector 32 is preferably located at thefocal point of the deflected beam 31.

The validating signal 39 is passed to a gate 27 along with the output 26of the zero crossing detector, and the gate produces an output 28 to bepassed to the machine only when the two outputs 26 and 39 indicate thata trigger signal and a validating signal have been generatedsimultaneously. The gate may be an AND gate as shown if both outputs 26and 39 are positive but may be an OR or a NOR gate if alternativeoutputs are desired from the zero crossing detector and the validatingcircuit.

Where it is considered undesirable to have stray light falling on thedetectors the modification illustrated in dotted lines in FIG. 1 may beadopted. In such a modification the light source is a laser and the beam6 is a polarised beam. The beam splitter 8 is a polarising beam splitterthe polarisation state of which is such as to allow transmission of thebeam through the beam splitter. A quarter wave plate 11 is provided inthe path of the beam so that the reflected beam from the surface 12,after passing twice through the wave plate will be reflected from theinclined surface 9 of the beam splitter onto the detectors. Thismodification will increase the intensity of the light reaching thedetectors since no part of the beam will be reflected at the beamsplitter, will thus eliminate stray reflected light from the beamsplitter, and will prevent any part of the reflected light from thesurface 12 being transmitted through the beam splitter back to thelaser.

Many other modifications may be made to the particular embodimentsdescribed above. For example, the light source 4 is preferably combinedwith a collimating lens so that the light beam is collimated beforereaching the half-silvered mirror. The light source may in fact be alaser which produces a coherent light beam. The lens system 10 may be asingle focussing lens.

In a simplified form of validating circuit the light source may bepulsed, whereby the ambient light levels inside the probe can bedetected by detectors ABC and D during the "off" periods of the lightsource and subtracted from the signals received during the "on" periods.By this means a true signal level can be obtained during the "on"periods and an appropriate threshold level set to produce the validatingsignal.

The four photo-diode detectors A, B, C, D may be replaced by aquadrature cell divided into four separate detectors and the prism 14would then be eliminated. The prism 4 may in any case be substituted bya diffraction grating.

In a simplified version of the FIG. 3 embodiment the guard detector 40and the focus detectors may be combined as elements of a photo-diodearray, four elements of which are designated detectors ABC and D. Alsothe gate 44 may be present as hardware in the electronics of the systemor as software in the machine software.

The machine on which the probe is mounted has not been described in anydetail but it is to be understood that the probe of the presentinvention could be used in conjunction with any co-ordinate measuringmachine or even a machine tool.

What I claim is:
 1. An optical measuring system for use with a machinecomprising a probe having a housing, a light source and a focussingdevice mounted within the housing for producing a light beam focussed ata point outside the housing, and a focus detection system for receivinglight reflected from a surface towards which the light beam is directed,characterised in that the light source and the focussing device aremounted in fixed relationship within the housing, the focus detectionsystem further comprising first and second detector means for producingtwo outputs indicative of the amount of reflected light falling ondifferent parts thereof and function means for receiving said twooutputs and for producing a focus error signal which has a zero valuewhen the light beam is in focus on the surface and a non-zero value whenthe light beam is out of focus on the surface, and wherein the focusdetection system further comprises a zero crossing detection circuit fordetecting the zero crossing points of the focus error signal and forproducing an output each time a zero crossing is detected, the outputbeing passed to the machine on which the probe is mounted.
 2. An opticalmeasuring device as claimed in claim 1, wherein the focus detectionsystem includes a beam splitter disposed in the path of the lightreflected from the surface to deflect at least a portion of thereflected light toward the first and second detector means.
 3. Anoptical measuring system as claimed in claim 1, wherein the focusdetection system further comprises a validating circuit for producing avalidating signal output only when the intensity of the light reflectedfrom the surface is greater than a pre-determined minimum level, andgate means are provided for receiving, as input signals, both the outputof the zero crossing detector circuit and the validating signal output,and for providing a further output only when both of said input signalsare present.
 4. An optical measuring system as claimed in claim 3,wherein the validating circuit includes means for providing an intensitysignal indicative of the total intensity of the light reflected from thesurface, and means for comparing the intensity signal with a thresholdlevel and for providing an output forming the validating signal outputwhen the intensity signal is greater than the threshold level.
 5. Anoptical measuring system as claimed in claim 4, wherein the means forproviding an intensity signal comprises a further beam splitterpositioned in the path of the light reflected from the surface and whichdeflects a known portion of the reflected light onto a second detectorsystem.
 6. An optical measuring system as claimed in claim 3, whereinthe validating circuit includes a photo-sensitive guard detector onwhich the first and second detector means are mounted, the guarddetector receiving light spilling around the first and second detectormeans and providing a noise signal indicative thereof, divider means fordetermining the ratio of the detector signal and the noise signal, andcomparator means for comparing the signal to noise ratio with athreshold value and which provides an output forming the validatingsignal output when the signal to noise ratio exceeds the thresholdvalue.
 7. An optical measuring system as claimed in claim 6, wherein afurther comparator is provided which compares the sum of the first andsecond detector means outputs with a threshold level and produces anoutput only when the sum of the first and second detector means outputsexceeds the threshold level, the output of the further comparator beingpassed as a third input signal to the gate means, said gate meansproducing an output only when all three input signals are present.
 8. Anoptical measuring system as claimed in claim 1, wherein the light sourceis a laser beam generator.
 9. An optical measuring system as claimed inclaim 8, wherein the laser beam produced by the laser beam generator ispolarised in one plane, the beam splitter is a polarising beam splitterthe polarising state of which is such as to transmit the laser beamtherethrough, and a quarter wave plate is disposed between the beamsplitter and the surface so that the beam passes through the quarterwave plate twice before it returns as reflected light from the surfaceand is thus polarised in a plane orthogonal to the original plane ofpolarisation and is reflected at the beam splitter towards the first andsecond detector means.
 10. An optical measuring system as claimed inclaim 2, wherein the focus detection system further comprises avalidating circuit for producing a validating signal output only whenthe intensity of the light reflected from the surface is greater than apre-determined minimum level, and gate means are provided for receiving,as input signals, both the output of the zero crossing detector circuitand the validating signal output, and for providing a further outputonly when both of said input signals are present.
 11. An opticalmeasuring system as claimed in claim 3 wherein the light source is alaser beam generator.